Solar on SIPs

The sample section of SIP, with the solar rack standoff mounted and ready to be tested for pull-through strength.

A Toggler Snaptoggle anchor is similar to a toggle bolt but with a stronger cross piece. (Note: Plastic straps are used only for initial placement.) Once in place, the anchor accepts a cap screw inserted from the opposite side of the SIP.

Intermediate

A structural insulated panel (SIP) roof has no embedded lumber in the structure, and therefore nothing substantial for attaching PV array and solar hot water mounts. Single or double lumber splines could be put into the panels, or I-joists at 4-foot intervals could be added. However, the thermal bridging that they would create are at odds with the goal of an energy-efficient building. As buildings get tighter and more insulated, thermal bridging plays a relatively larger role in energy loss.

One solution for mounting solar equipment to a SIP roof is to drill completely through the roof, passing a threaded rod through to the bottom skin and placing a large washer under the nut. Since most SIP roofs are vaulted on the inside, though, few homeowners would be OK with seeing the nuts and washers on their ceilings.

On a recent SIP project in Hood River, Oregon, the PV contractor asked for test results for fastener pull-out so he could design an appropriate rack system. An independent test at Rigging Products in Portland, Oregon, provided the figures he needed. The first test, using hollow-wall anchors, gave a result of 405 pounds to failure. Dividing by three (which accounts for the industry standard safety factor) gives a working load of 135 pounds. Failure occurred as the hollow-wall anchor folded up and pulled through the anchor holes. In an area with 100 mph wind gusts, we did not think a hollow-wall anchor would hold up.

In the second test, two 3/8-inch, high-performance toggle bolts were used. Toggler brand bolts were used with cap screws instead of the original machine screws since the holes in the standoff were 3/8 inch. In hindsight, I would have used 5/16-inch bolts to give a little wiggle room for alignment. In this test, the failure of the SIP—when the OSB skin cracked—occurred at 1,105 pounds, or a working load of 368 pounds.

Comments (5)

This test covers pull out strength, but what about compression? We need to be sure the panels can support 30 pounds per square foot snow load. With these attachment points, that load is going to be concentrated.

A standing seem metal roof and S5! mounts on a SIP roof should also be considered. The metal will last at least as long as the panels, the load is distributed broadly across the roof without bridging, and it works well in high wind conditions (see S5! website for load tests since there are many factors). Of course, some people object to the aesthetics of a metal roof -- to each his own.

Hi Lawrence,
Thanks for posting. Yes, if you check out the caption to the second photo you will see that it is a Toggler brand Snaptoggle:

"A Toggler Snaptoggle anchor is similar to a toggle bolt but with a stronger cross piece. (Note: Plastic straps are used only for initial placement.) Once in place, the anchor accepts a cap screw inserted from the opposite side of the SIP."